According to the pilot, while approaching the destination airport, he entered the traffic pattern by crossing the runway approximately 850 feet above ground level. He performed a "wing over," then turned back over the runway and began a turn to the left downwind leg of the traffic pattern. During the turn, the rudder "stuck" and the airplane entered a left spiraling descent. The pilot added full power, pushed the control stick forward, and attempted to apply right rudder. He attempted to dislodge the rudder by reaching for the passenger rudder pedal underneath and behind his seat, but was unsuccessful. Around 100 feet about ground level, the airplane exited from the spiral, continued to descend into the ground, then came to rest inverted, resulting in substantial damage to the wings and fuselage. According to a witness, the airplane was about 400 feet above ground level when it entered a steep left turn, and then entered a left hand spin. After one and one half revolutions the pilot recovered from the spin into a dive. The pilot was unable to recover from the dive before ground impact. A subsequent examination of the wreckage performed by a Federal Aviation Administration inspector verified full freedom of movement of the rudder and rudder pedals. There was no evidence of any pre-impact mechanical anomalies with the airplane.